This invention relates to a composite shaft of circular section which comprises two or more axial segments. In particular, the invention relates to a shaft in which at least one interface of adjacent segments is formed between segments which have different mechanical properties, thermal properties, or both.
The shaft may rotate and its temperature may change in the operation of the machine of which the shaft is a component. If at the interface of dissimilar segments the segments were unconstrained one with respect to the other then the radial displacements to one side of the interface due to thermal expansion or to centrifugal strain or to both would be different from the radial displacement to the other side.
In the following, two kinds of segment are differentiated by the fact that one kind has lesser expansion and the other kind has greater expansion, even though in practice, at an interface between the two kinds of segment their expansions may be constrained to be equal.
It is a requirement that the axial segments should remain sufficiently coaxial when the machine of which the shaft is a component goes through its operating cycles. The term sufficiently coaxial is defined as being coaxial to such an accuracy that a greater accuracy would not produce any change of practical importance in the behaviour in operation of the machine.
It cannot be relied upon that the sliding at their interface arising from the radial differential expansion between dissimilar segments will be axially symmetric and means to impose that symmetry have to be provided to ensure that the initial concentricity of the segments at their interface is not degraded by frictional ratcheting.
For that purpose and to resist other disturbing forces which might arise it is well known in the art to spigot similar or dissimilar segments together. With dissimilar segments the male member of the spigot is formed on the segment which will undergo less radial expansion in operation of the machine and the female member is bored to a lesser diameter than that of the male member. The segments are assembled by expanding the diameter of the female member for instance by heating that member so that the diameter of its bore in the hot condition is greater than or equal to the diameter of the male member in which instance the spigot is left pre-stressed on cooling. The pre-stress will decrease in operation of the machines. The initial pre-stress is calculated so that some pre-stress remains at all times in operation of the machine.
An object of the present invention is to minimise the pre-stress which has to be imposed. This can be a particularly pertinent object when the segment of lesser radial expansion is of brittle material such as a ceramic.
According to the present invention there is provided a composite shaft comprising at least two axial segments of dissimilar materials, the segments being held together by a central tie-bar, wherein a segment of material least subject to radial expansion is provided with a coaxial male member extending axially from its end, and the cooperating segment subject to greater radial expansion is provided with a female skirt shrunk onto the male member in the assembly of the shaft and also being provided with an annular upstand, at or towards its peripheral edge and directed towards the cooperating segment, the upstand being held by the compressive force of the tie-bar always in full annular contact with its cooperating segment.
The male member may be of the smallest practicable diameter and the upstand may be of as large a diameter as possible.
The requirement that the segments should remain sufficiently coaxial at all times may be divided into two sub-requirements of firstly a requirement that at the interface of the segments there should be a sufficient stiffness against transverse shear and secondly that there should be a sufficient stiffness against bending. This division permits each sub-requirement to be satisfied by a distinct feature of a junction between dissimilar segments. The first sub-requirement can be satisfied at a relatively small diameter where the differential expansion to be contained is correspondingly small and the second at a larger diameter which is conducive to a high resistance to bending.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: